The traditional way of motor vehicle sheet blanking is die blanking, the equipments for which include a transfer unit, a press unit and a stack unit. Usually, sheets of a simple shape (such as rectangular, trapezoid and arc) can be cut out by a swing shear, while sheets with complicated profiles should be made by die blanking. A die blanking line is required to be provided with different blanking dies corresponding to the sizes and shapes of specific motor vehicle sheets such that various sheets can be produced by changing the dies.
The blanking line for motor vehicle sheet is of high production efficiency, at 4-6 million pieces per year. Calculating by 3800 hours per year, the production is 25 pieces per minute.
With the rapid development of high power laser technology, many academic institutions and manufacturers begin the research of blanking for motor vehicle sheets based on fiber laser. Both of Automatic Feed Company in USA and Air Liquide in France have, in the publication information, published the motor vehicle sheet blanking technology based on laser cutting. The solution of the former company puts forward two blanking modes on basis of laser technology: one is strip continuous-feed blanking, and the other is step blanking.
1. Strip Continuous-Feed Blanking
Nesting the sheets, and according to the number n of the provided laser cutting heads, dividing the cutting line of each sheet into n segments on average; when the sheet enters a first cutting region, cutting along a first part of line segment or curve such as one semi-circle; when the sheet enters a second cutting region, cutting along a second part of the line segment or curve such as the other semi-circle, with circle scraps falling onto a scrap belt underneath; when the sheet enters a third cutting region, cutting along a third part of the line segment or curve, with edge scraps falling onto the scrap belt underneath; when the sheet enters a fourth cutting region, cutting along a fourth part of the line segment or curve, with edge scraps falling onto the scrap belt underneath; after the sheet passes through the pinch rollers, the fifth cutting head cuts transversely the last connecting parts between sheets; the sheets come into a stacking region and get stacked.
2. Step Blanking
Nesting the sheets; cutting the strip into group sheets, and transferring them into a cutting position; a group sheet enters the cutting region and stops; adjusting longitudinally the two groups of conveyor rollers before and after the cutting position until the shape of scrap is cut out; when finishing the cutting of the first area of the sheet, scraps fall into a scrap conveyor belt; the sheet moves into a second cutting area and stops; adjusting the two groups of transfer rollers before and after the cutting position longitudinally until the shape of scrap is cut out; when finishing the cutting of the second area of the sheets, scraps fall into the scrap conveyor belt; the sheet moves into a third cutting area and stops; adjusting the two groups of transfer rollers before and after the cutting position longitudinally until the shape of scrap is cut out; when finishing the cutting of the third area of the sheets, scraps fall into the scrap conveyor belt; after the sheet is cut, it is transferred into a stacking region through a conveying device. The subsequent sheets are cut in the same way.
Additionally, Air Liquide proposes a scrap blanking solution with transverse backing and adjustment for sheets.
Automatic Feed Company together with Air Liquide proposes a technical solution of step-type laser blanking, which generally adopts the layout of Air Liquide and the scrap processing by longitudinal adjustment of Automatic Feed Company. In this solution, 3 robots with a seventh shaft are utilized to transfer group sheets and finished sheets, and to stack sheets. 5 laser cutting units are provided, two of which is used for making group sheets, and another three for cutting single sheets in different areas. Besides, there are further two stations as buffering stations for discharging scraps.
In the traditional swing shear blanking, the front end of a sheet is free of clamping or fixing, and the back end thereof is transferred by pinch rollers in a stepping way. The base of the sheet is supported by wheels, and during blanking, a return lock device is used.
As for the strip conveyance with continuous blanking proposed by Automatic Feed Company, there are provided with pinch rollers at both front and back ends in the cutting region, and sheets are fed continuously at a preset speed. Under the sheets there is provided with a scrap conveyor device, onto which the cut scraps fall. The last cut is performed after the pinch rollers at the back end.
The technical problems of the conveying way above cover the following aspects.
1. Shape cutting is based on speed control strategy
Conventionally, the displacement control strategy is used for cutting operation with high requirements on dimension and form errors, so as to guarantee the geometric accuracy of the shape. However, in the continuous blanking of Automatic Feed Company, the speed control strategy is utilized, which is on the basis that the speed of the pinched sheets is precise and the Cartesian coordinate robots are cutting according to the predetermined traces and speeds. The speed deviation of the pinch rollers and the sheets may bring about errors of cut shapes, which in turn, results in that the kerfs cannot to be engaged together, and the scraps cannot drop. As a lot of cutting machines are working at the same time, even if using a speed measuring device (for example, a laser velocimeter), it is still difficult to ensure the kerfs made from the different cutting machines to be engaged well.
2. Three equipments have the same linear speed. The solution of Automatic Feed Company requires that the speeds of the pinch rollers and the two circular pin-type conveyor belt which support sheets before and after the pinch rollers, should be the same, otherwise the pin-type support parts on the belt may scrape the sheets.
3. The pins on the pin-type conveyor belts are prone to being cut, and little scraps cut from sheets may get stuck in the conveyor belt.
4. The sheet shape in the cutting region cannot be guaranteed.
When continuously conveyed and cut, the sheets are present in the cutting regions. In this solution, there are three cutting regions and in addition, a cutting-off region after the pinch rollers, in each of which a different area of the sheet is cut. The sheets assume a state of being pinched and tensioned, but there is great difference among transverse tensions at different segments in a sheet, resulting in that the sheet shape in the cutting region cannot be guaranteed, thereby affecting the cutting quality. When cutting, the distance between the cutting head and the sheet surface is 0.5 mm˜1 mm.
5. It is difficult for the sheets to pass through the back pinch rollers.
When a sheet is passing through the back pinch rollers during continuous feeding and cutting, it is not only prone to straining the pinch rollers, but also causing sheets stuck or even broken.
6. Off-center sheets affect shape cutting.
Due to that the sheets are transferred through pinch rollers and the distance between two sheets is above 10 m, the off-center phenomenon may lead to making batches of waste product in cutting operation. Thus, in the solution there must be a centering device.
7. High requirement on the engagement of the kerfs
For the reason that the sheets are cut by a plurality of cutting machines in different regions, it is necessary for the latter cutting to engage with the former kerfs, so as to ensure scraps to drop. As the kerfs engagement is performed when the sheets are moving, there are high requirements on the movement of the pinch rollers, the stepping progress of the robots and the shape of the sheet, thereby it costs a lot on the technology and equipments.
8. Cutting along the trace of a sheet is performed in relative movement.
While the cutting devices are Cartesian coordinate robots with 3 degrees of freedom, owing to that sheets move in a single direction, it is hard to cut along the trace of the sheet or it needs a large quantity of empty stroke to guarantee.
9. The malfunction of a single cutting machine may cause the whole production line to halt.
As one sheet is cut by different cutting machines on different areas thereof, the malfunction of one of the cutting machines may affect the operation of the whole production line. When one cutting machine breaks down, if it is needed to finish the work of the production line, a complicated cutting control strategy and a hardware technology have to be utilized for cutting in the abnormal situation.
The laser step blanking solution proposed by Automatic Feed Company and Air Liquide together is better on operability than the continuous blanking solution proposed by Automatic Feed Company, while the difficulty thereof is the scrap blanking and the yield during unit time of the whole production line.